fix: handle null ratingKey in folder browsing

lib/models/plex_metadata.dart

@@ -9,6 +9,9 @@ import '../utils/global_key_utils.dart';
 
 part 'plex_metadata.g.dart';
 
+Object? _readRatingKey(Map json, String key) =>
+    json['ratingKey'] ?? json['key'] ?? '';
+
 int? _flexibleInt(Object? v) => switch (v) {
       num n => n.toInt(),
       String s => int.tryParse(s),
@@ -57,6 +60,7 @@ enum PlexMediaType {
 
 @JsonSerializable()
 class PlexMetadata with MultiServerFields {
+  @JsonKey(readValue: _readRatingKey)
   final String ratingKey;
   final String key;
   final String? guid;

lib/models/plex_metadata.g.dart

@@ -7,7 +7,7 @@ part of 'plex_metadata.dart';
 // **************************************************************************
 
 PlexMetadata _$PlexMetadataFromJson(Map<String, dynamic> json) => PlexMetadata(
-  ratingKey: json['ratingKey'] as String,
+  ratingKey: _readRatingKey(json, 'ratingKey') as String,
   key: json['key'] as String,
   guid: json['guid'] as String?,
   studio: json['studio'] as String?,